A. Field of the Invention
This invention relates to a built-in wheel speed sensor structure for a vehicle having a mounted brake control device.
B. Description of the Prior Art
When an anti-lock brake control device is mounted to a car employing a drum brake, the present device relates to a built-in wheel speed sensor structure for a car wherein an electro-magnetic wheel speed sensor is internally fitted to and supported by the drum brake. Additionally, an internal gear-like sensor rotor corresponding to the sensor is integrally fixed to a rotary wheel in such a manner as to face the sensor.
Remarkable progress in the studies on friction characteristics between a running road surface and a tire of a car have been made in recent years. For example, we know that the friction characteristics vary according to a slip ratio between the road surface and the tire and attain the highest level when the slip ratio is about 20%.
An anti-lock brake system has been developed which controls the slip ratio to within about 20% at the time of braking the car. The anti-lock brake system has been fitted to several models of cars in order to prevent side slipping of the tires during abrupt braking action.
In general, the above-described anti-lock brake system comprises a wheel speed sensor, a computer and an actuator. The wheel speed sensor is disposed in the proximity of a rotary member, such as a wheel or a transmission, in order to detect the car speed based upon a car speed signal and the associated revolutions of the rotary member. The wheel speed signal detected by the wheel speed sensor is then sent to, and controlled by, the computer as a brake oil pressure instruction signal adapted to the wheel speed and subsequently sent to the actuator. The actuator in turn receives the signal from the computer, increases or decreases the brake oil pressure and controls the braking force to a suitable level.
The wheel speed sensor is supported at a fixed position of a car body in such a manner as to face an internal gear-like sensor rotor which is integrally fixed to the rotary member described above and which detects the change of the magnetic flux of a magnetic iron plate at the tip of the sensor due to the revolution of the sensor rotor as induction electromotive force of a coil which is incorporated therein.
In general, the wheel speed sensor and the sensor rotor are fitted to a differential-gear device or an output shaft of a transmission gear box and the like in front engine-rear drive type cars, but are seldom fitted to the rear part of the car body in front-engine-front-drive type cars (hereinafter referred to as "FF cars"). When they are fitted to the rear part of the car body, their fitting positions are unavoidably limited to the portions near the axle.
Accordingly, it presently is customary to fit the wheel speed sensor to a flange end of a bearing case which is built in a parking brake and to integrally fix the sensor rotor inside the drum in such a manner as to correspond to the wheel speed sensor thus fitted.
However, when the wheel speed sensor is fitted to a disc brake which is also equipped with a drum brake for parking, the prior art example described above, in which the wheel speed sensor is fitted to the end flange of the bearing case, involves the following problem. Namely, since a brake shoe which undergoes displacement in the proximity of the outer periphery of the bearing case is externally fitted, a predetermined gap must be secured between the brake shoe and the wheel speed sensor. When this gap is obtained by forming a notch on the brake shoe, the brake shoe itself is weakened.
Even if the reduction in strength of the brake shoe itself is eliminated by forming the notch on the brake shoe and then somehow reinforcing the brake shoe, the portion at which the wheel speed sensor is close to the center of the axle necessarily means that the sensor rotor which corresponds to the sensor must also be disposed close to the center of the axle. As a result, the sensor rotor would overlap with the position of fitting bolts for the bearing case, thus making fastening impossible when an impact wrench or the like is inserted through a service hole bored in the brake drum.